Re: [PATCH V4 16/27] ublk: add new feature UBLK_F_BATCH_IO

Caleb Sander Mateos
2025-12-02T16:05:29Z

On Mon, Dec 1, 2025 at 5:44 PM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Mon, Dec 01, 2025 at 01:16:04PM -0800, Caleb Sander Mateos wrote:
> > On Thu, Nov 20, 2025 at 6:00 PM Ming Lei <ming.lei@redhat.com> wrote:
> > >
> > > Add new feature UBLK_F_BATCH_IO which replaces the following two
> > > per-io commands:
> > >
> > > - UBLK_U_IO_FETCH_REQ
> > >
> > > - UBLK_U_IO_COMMIT_AND_FETCH_REQ
> > >
> > > with three per-queue batch io uring_cmd:
> > >
> > > - UBLK_U_IO_PREP_IO_CMDS
> > >
> > > - UBLK_U_IO_COMMIT_IO_CMDS
> > >
> > > - UBLK_U_IO_FETCH_IO_CMDS
> > >
> > > Then ublk can deliver batch io commands to ublk server in single
> > > multishort uring_cmd, also allows to prepare & commit multiple
> > > commands in batch style via single uring_cmd, communication cost is
> > > reduced a lot.
> > >
> > > This feature also doesn't limit task context any more for all supported
> > > commands, so any allowed uring_cmd can be issued in any task context.
> > > ublk server implementation becomes much easier.
> > >
> > > Meantime load balance becomes much easier to support with this feature.
> > > The command `UBLK_U_IO_FETCH_IO_CMDS` can be issued from multiple task
> > > contexts, so each task can adjust this command's buffer length or number
> > > of inflight commands for controlling how much load is handled by current
> > > task.
> > >
> > > Later, priority parameter will be added to command `UBLK_U_IO_FETCH_IO_CMDS`
> > > for improving load balance support.
> > >
> > > UBLK_U_IO_GET_DATA isn't supported in batch io yet, but it may be
> >
> > UBLK_U_IO_NEED_GET_DATA?
>
> Yeah.
>
> >
> > > enabled in future via its batch pair.
> > >
> > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > ---
> > > drivers/block/ublk_drv.c | 58 ++++++++++++++++++++++++++++++++---
> > > include/uapi/linux/ublk_cmd.h | 16 ++++++++++
> > > 2 files changed, 69 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > > index 849199771f86..90cd1863bc83 100644
> > > --- a/drivers/block/ublk_drv.c
> > > +++ b/drivers/block/ublk_drv.c
> > > @@ -74,7 +74,8 @@
> > > | UBLK_F_AUTO_BUF_REG \
> > > | UBLK_F_QUIESCE \
> > > | UBLK_F_PER_IO_DAEMON \
> > > - | UBLK_F_BUF_REG_OFF_DAEMON)
> > > + | UBLK_F_BUF_REG_OFF_DAEMON \
> > > + | UBLK_F_BATCH_IO)
> > >
> > > #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
> > > | UBLK_F_USER_RECOVERY_REISSUE \
> > > @@ -320,12 +321,12 @@ static void ublk_batch_dispatch(struct ublk_queue *ubq,
> > >
> > > static inline bool ublk_dev_support_batch_io(const struct ublk_device *ub)
> > > {
> > > - return false;
> > > + return ub->dev_info.flags & UBLK_F_BATCH_IO;
> > > }
> > >
> > > static inline bool ublk_support_batch_io(const struct ublk_queue *ubq)
> > > {
> > > - return false;
> > > + return ubq->flags & UBLK_F_BATCH_IO;
> > > }
> > >
> > > static inline void ublk_io_lock(struct ublk_io *io)
> > > @@ -3450,6 +3451,41 @@ static int ublk_validate_batch_fetch_cmd(struct ublk_batch_io_data *data,
> > > return 0;
> > > }
> > >
> > > +static int ublk_handle_non_batch_cmd(struct io_uring_cmd *cmd,
> > > + unsigned int issue_flags)
> > > +{
> > > + const struct ublksrv_io_cmd *ub_cmd = io_uring_sqe_cmd(cmd->sqe);
> > > + struct ublk_device *ub = cmd->file->private_data;
> > > + unsigned tag = READ_ONCE(ub_cmd->tag);
> > > + unsigned q_id = READ_ONCE(ub_cmd->q_id);
> > > + unsigned index = READ_ONCE(ub_cmd->addr);
> > > + struct ublk_queue *ubq;
> > > + struct ublk_io *io;
> > > + int ret = -EINVAL;
> >
> > I think it would be clearer to just return -EINVAL instead of adding
> > this variable, but up to you
> >
> > > +
> > > + if (!ub)
> > > + return ret;
> >
> > How is this case possible?
>
> Will remove the check.
>
> >
> > > +
> > > + if (q_id >= ub->dev_info.nr_hw_queues)
> > > + return ret;
> > > +
> > > + ubq = ublk_get_queue(ub, q_id);
> > > + if (tag >= ubq->q_depth)
> >
> > Can avoid the likely cache miss here by using ub->dev_info.queue_depth
> > instead, analogous to ublk_ch_uring_cmd_local()
>
> OK.
>
> >
> > > + return ret;
> > > +
> > > + io = &ubq->ios[tag];
> > > +
> > > + switch (cmd->cmd_op) {
> > > + case UBLK_U_IO_REGISTER_IO_BUF:
> > > + return ublk_register_io_buf(cmd, ub, q_id, tag, io, index,
> > > + issue_flags);
> > > + case UBLK_U_IO_UNREGISTER_IO_BUF:
> > > + return ublk_unregister_io_buf(cmd, ub, index, issue_flags);
> > > + default:
> > > + return -EOPNOTSUPP;
> > > + }
> > > +}
> > > +
> > > static int ublk_ch_batch_io_uring_cmd(struct io_uring_cmd *cmd,
> > > unsigned int issue_flags)
> > > {
> > > @@ -3497,7 +3533,8 @@ static int ublk_ch_batch_io_uring_cmd(struct io_uring_cmd *cmd,
> > > ret = ublk_handle_batch_fetch_cmd(&data);
> > > break;
> > > default:
> > > - ret = -EOPNOTSUPP;
> > > + ret = ublk_handle_non_batch_cmd(cmd, issue_flags);
> >
> > We should probably skip the if (data.header.q_id >=
> > ub->dev_info.nr_hw_queues) check for a non-batch command?
>
> It is true only for UBLK_IO_UNREGISTER_IO_BUF.

My point was that this relies on the q_id field being located at the
same offset in struct ublksrv_io_cmd and struct ublk_batch_io, which
seems quite subtle. I think it would make more sense not to read the
SQE as a struct ublk_batch_io for the non-batch commands.

Best,
Caleb

>
> >
> > > + break;
> > > }
> > > out:
> > > return ret;
> > > @@ -4163,9 +4200,13 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
> > >
> > > ub->dev_info.flags |= UBLK_F_CMD_IOCTL_ENCODE |
> > > UBLK_F_URING_CMD_COMP_IN_TASK |
> > > - UBLK_F_PER_IO_DAEMON |
> > > + (ublk_dev_support_batch_io(ub) ? 0 : UBLK_F_PER_IO_DAEMON) |
> >
> > Seems redundant with the logic below to clear UBLK_F_PER_IO_DAEMON if
> > (ublk_dev_support_batch_io(ub))?
>
> Good catch.
>
> >
> > > UBLK_F_BUF_REG_OFF_DAEMON;
> > >
> > > + /* So far, UBLK_F_PER_IO_DAEMON won't be exposed for BATCH_IO */
> > > + if (ublk_dev_support_batch_io(ub))
> > > + ub->dev_info.flags &= ~UBLK_F_PER_IO_DAEMON;
> > > +
> > > /* GET_DATA isn't needed any more with USER_COPY or ZERO COPY */
> > > if (ub->dev_info.flags & (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY |
> > > UBLK_F_AUTO_BUF_REG))
> > > @@ -4518,6 +4559,13 @@ static int ublk_wait_for_idle_io(struct ublk_device *ub,
> > > unsigned int elapsed = 0;
> > > int ret;
> > >
> > > + /*
> > > + * For UBLK_F_BATCH_IO ublk server can get notified with existing
> > > + * or new fetch command, so needn't wait any more
> > > + */
> > > + if (ublk_dev_support_batch_io(ub))
> > > + return 0;
> > > +
> > > while (elapsed < timeout_ms && !signal_pending(current)) {
> > > unsigned int queues_cancelable = 0;
> > > int i;
> > > diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
> > > index cd894c1d188e..5e8b1211b7f4 100644
> > > --- a/include/uapi/linux/ublk_cmd.h
> > > +++ b/include/uapi/linux/ublk_cmd.h
> > > @@ -335,6 +335,22 @@
> > > */
> > > #define UBLK_F_BUF_REG_OFF_DAEMON (1ULL << 14)
> > >
> > > +
> > > +/*
> > > + * Support the following commands for delivering & committing io command
> > > + * in batch.
> > > + *
> > > + * - UBLK_U_IO_PREP_IO_CMDS
> > > + * - UBLK_U_IO_COMMIT_IO_CMDS
> > > + * - UBLK_U_IO_FETCH_IO_CMDS
> > > + * - UBLK_U_IO_REGISTER_IO_BUF
> > > + * - UBLK_U_IO_UNREGISTER_IO_BUF
> >
> > Seems like it might make sense to provided batched versions of
> > UBLK_U_IO_REGISTER_IO_BUF and UBLK_U_IO_UNREGISTER_IO_BUF. That could
> > be done in the future, I guess, but it might simplify
> > ublk_ch_batch_io_uring_cmd() to only have to handle struct
> > ublk_batch_io.
>
> Agree, and it can be added in future.
>
>
>
>
> Thanks,
> Ming
>